Measuring device using an indirect measurement of permittivity

ABSTRACT

Measuring device is presented employing indirect measurement of permittivity and including two electrically conductive bodies respectively constituting a measuring probe and a reference probe, electrical power supply means adapted to deliver a DC electrical voltage of controlled amplitude, an integrator stage including a capacitor switching system and control means adapted to define a cyclic series of two sequences at a controlled frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sensors.

To be more precise, the present invention relates to a measuring deviceusing an indirect measurement of permittivity between two electricallyconductive bodies respectively forming a measuring probe and a referenceprobe.

2. Description of Related Art

There are many sensors based on measuring permittivity or of thecapacitive type.

In particular, there are many devices in which a measuring capacitor isconnected to an oscillator circuit so that the output frequency of thatcircuit depends on the capacitance of the measuring capacitor, enablingdetermination of a parameter influencing the permittivity of thecapacitor, for example the level of a liquid contained in a tank inwhich the measuring capacitor is placed (see, for example, documentsWO-A-98/02718, DE-A-4312432 and DE-A-4434338).

Various devices have also been proposed which comprise a measuringcapacitor connected to the input of an integrator stage (see, forexample, DE-A-3413849 and Journal of Physics E. Scientific Instruments,vol. 22, no 2, 1989). However, these devices have not provedsatisfactory until now and for this reason have not been usedindustrially.

The documents FR-A-2205669, FR-A-2605731, FR-A-2447555, FR-A-2737297,EP-A-0378304 and EP-A-0644432 describe various devices for measuring thetime to charge or discharge a measuring capacitor influenced by theparameter to be detected.

Other capacitive measuring devices are described in the documentsFR-A-2763124, FR-A-1152556 and U.S. Pat. No. 3,706,980.

The object. of the present invention is to propose new detector means ofvery high sensitivity.

Another object of the present invention is to propose detector meanssuited to many applications.

BRIEF SUMMARY OF THE INVENTION

The above targets are achieved in the context of the present inventionby a measuring device employing indirect measurement of permittivity andcharacterized in that it includes two electrically conductive bodiesrespectively constituting a measuring probe and a reference probe,electrical power supply means adapted to deliver a DC electrical voltageof controlled amplitude, an integrator stage including a capacitorswitching system and control means adapted to define a cyclic series oftwo sequences at a controlled frequency, namely a first sequence duringwhich the electrical power supply means are connected to the measuringprobe to apply an electric field between the measuring probe and thereference probe and accumulate electrical charge on the measuring probeand a second sequence during which the electrical power supply means aredisconnected from the measuring probe which is connected to a summationpoint of the integrator stage to transfer its charge into the integratorstage and obtain at the output thereof a signal representative of thepermittivity between the measuring probe and the reference probe.

According to another and advantageous feature of the present inventionthe control means are adapted to apply a stepped voltage to themeasuring probe.

According to another and advantageous feature of the present inventionthe integrator stage includes an operational amplifier, a firstintegrator capacitor of high capacitance constituting a feedbackcapacitor of said amplifier and a second capacitor switched between theoutput and the input of the operational amplifier at the timing rate ofthe sequences controlled by the control means.

According to another and advantageous feature of the present inventionthe device includes means for applying a null average voltage to themeasuring probe.

According to another and advantageous feature of the present inventionthe operational amplifier receives on a second input opposite thatadapted to be connected sequentially to the measuring probe a voltage ofopposite sign to the voltage applied by the power supply means to themeasuring probe.

According to another and advantageous feature of the present inventionthe amplitude of the voltage applied to the second input of theoperational amplifier is equal to p.E where E is the amplitude of thevoltage applied to the measuring probe during the time T1 and p is theduty factor between the two sequences T1 and T2, i.e. T1=p.T2.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, aims and advantages of the present invention will becomeapparent on reading the following detailed description with reference tothe accompanying drawings, which are given by way of non-limitingexample, and in which:

FIG. 1 is a diagrammatic representation of the structure of a measuringdevice constituting a first embodiment of the present invention,

FIG. 2 shows a variant of this device including an auxiliary measuringprobe providing a permittivity correction,

FIG. 3 shows a preferred embodiment of the present invention includingmeans for applying a null average voltage to the measuring probe,

FIG. 4 is a schematic representation of an embodiment of the deviceaccording to the present invention adapted to determine a spectrum of ananalysed product,

FIGS. 5 to 13 show various embodiments of devices according to thepresent invention designed to detect the level in a tank,

FIG. 14 shows an application to the detection of a product in a pipe,

FIG. 15 shows an application to intruder detection,

FIG. 16 shows an application to detecting the presence and or theposition of a user on a seat,

FIG. 17 shows an application to detecting passage through a doorway,

FIG. 18 shows an application to detecting objects conveyed by a conveyorbelt,

FIG. 19 shows an application to a keypad,

FIG. 20 shows an application to a pressure sensor, and

FIG. 21 shows an application to a pneumatic pressure detector.

DETAILED DESCRIPTION OF THE INVENTION

As previously indicated, the device shown in FIG. 1 essentiallycomprises:

a measuring probe 100,

a reference probe 200,

electrical power supply means 300,

control means 400 and

an integrator stage 500.

The measuring probe 100 and the reference probe 200 are each formed ofan electrically conductive body. The latter are spaced to define atleast one dielectric medium between them. Various examples of suchprobes 100 and 200 are described in further detail later.

The power supply means 300 are adapted to deliver a DC electricalvoltage of controlled amplitude E.

The control means 400 are adapted to define a cyclic series of twosequences at a control frequency f. During a first sequence of durationT1 the electrical power supply means 300 are connected to the measuringprobe 100 to apply an electric field between the measuring probe 100 andthe reference probe 200 and to accumulate electrical charge on themeasuring probe 100. The capacitance between the measuring probe 100 andthe reference probe 200 is charged during a very short time perioddefined by τ=ε/γ, where ε is the permittivity and γ is the conductivityof the medium between the two probes 100 and 200. Then, during a secondsequence of duration T2, the electrical power supply means 300 aredisconnected from the measuring probe 100, which is connected to theinput of the integrator stage 500. The control means 400 therefore applybetween the measuring probe 100 and the reference probe 200 alow-intensity pulsed electric field of controlled amplitude andduration. This method of deactivating the electric field applied to themeasuring probe 100 (by a sudden interruption, as opposed to aprogressive reduction in the voltage) immobilizes the electrical chargepreviously accumulated on the measuring probe 100. The electricalcharges on the measuring probe 100 at the end of the first sequence T1are proportional to the value of the permittivity between the probes 100and 200. These charges are transferred into the integrator stage 500 (tobe more precise into the capacitor switching system) in a mannerdescribed in more detail later. There is therefore obtained at theoutput of the stage 500 a signal which is representative of thepermittivity between the measuring probe 100 and the reference probe200.

In the embodiment shown in FIG. 1, the control means 400 include atimebase 410 formed for example by an oscillator and two two-wayswitches 420, 430.

The switch 420, controlled by the timebase 410, is adapted to connectthe measuring probe 100 alternately to the power supply means 300 duringthe sequences T1 and then to the input of the integrator stage 500during the sequences T2.

If necessary, a capacitive injector 110 can be inserted between themeasuring probe 100 and the switch 420.

Similarly, if necessary, a resistive injector 120 can be insertedbetween the measuring probe 100 and the reference probe 200. Theresistance of the injector 120 must be very high in order not to cause aleakage current that could interfere with the measurement.

The integrator stage 500 shown in FIG. 1 includes an operationalamplifier 510 and two capacitors whose values are known: one capacitor520 is connected between the inverting input and the output of theamplifier 510 and constitutes a feedback capacitor and the othercapacitor 530 has a first terminal connected to the same potential asthe reference probe 200 and the non-inverting input of the amplifier510, i.e. the ground of the circuit, and a second pin controlled by thecontrol means 400 so that it is connected alternately to the invertinginput of the amplifier 510 during the time T2 to transfer the chargesaccumulated on the measuring probe 100 into the capacitor 530 and thento the output of the amplifier 510 during the time T1 to deliver anelectric signal proportional to the charge accumulated on the measuringprobe 100.

The feedback capacitor 520 connected to the operational amplifier 510converts it into an integrator. It gets a DC voltage proportional to thecharges developed on the measuring probe 100 and thereby preventsunwanted interference voltages.

The integration capacitor 520 typically has a capacitance at least 1000times the capacitance of the switching capacitor 530 in the case of ananalog measurement requiring high accuracy, for example a levelmeasurement.

As an alternative to this, however, for a faster measurement, C520 canbe of the same order of magnitude as C530, for example C520 can be twoto three times C530. In this case the measurement is not so accurate butcan track fast changes.

Switching the measuring probe 100 to a summing point S of an operationalamplifier 510 has two advantages. Because the summing point (S) of theamplifier 510 has a null virtual impedance, the probe 100 is totallydischarged of the accumulated charge during the time T2, returning it toa null potential for a new measurement during the time T1. Also, all ofthe charge is transferred into the capacitor switching system 520, 530without losses, making the measurement perfectly linear.

In more detail, the integrator stage 500 operates in the followingmanner.

Assume that initially the integration capacitor C520, the switchingcapacitor C530 and the capacitor Cs formed between the measuring probe100 and the reference probe 200 are all totally discharged, so that:

QC 520=0

QC 530=0

QCs=0

During the first sequence T1, the capacitor Cs is charged to the supplyvoltage delivered by the module 300, which here is assumed to be equalto −E.

Thus at the end of the sequence T1:

QCs=−E.Cs

QC 520=0

QC 530=0

During the next sequence T2, the charge is transferred from Cs to C520;because the charge is conserved and Cs and C530 are connected to theinverting input of the operational amplifier 510 with a null virtualimpedance:

 −E.Cs=V _(s2) .C 520

where V_(s2) is the output voltage of the operational amplifier 510during the sequence T2.

During the next sequence T1, the two capacitors C520 and C530 areconnected in parallel. Thus:

V _(s) =V _(s2) .C 520/(C 520+C 530)=QC 530/C 530=QC 520/C 520

so:

QC 530=[V _(s2) .C 520/(C 520+C 530)].C 530

i.e.

QC 530=[V _(s2)/(1+C 530/C 520)].C 530

Thus if:

C 520=nC 530>>C 530

then:

QC 530˜V _(s2) .C 530

In the next sequence T2, the charges on C530 are the opposite of thecharges on Cs. The remainder of the charges from Cs are transferred intoC520, etc.

The output voltage V_(s) of the operational amplifier 510 increasesprogressively to a balancing voltage

V _(s) balancing=QC 530/C 530

such that:

QC 530=V _(s) balancing.C 530−E.Cs

Thus after x iterations the device reaches equilibrium at the summationpoint. The charges QC530 from C530 balances the charges from the probeCs.

Immediately any variation of capacitance Cs is detected, the additional(or lost) charge on Cs charges (or discharges) the capacitor C520.

Thus under steady state conditions the switching capacitor C530 balancesvariations of the charge on the probe Cs.

The foregoing operation presupposes that the switches 420, 430 areperfectly synchronized, i.e. in particular that at the start of thesequences T2 the switching capacitor C530 and the measuring probe Cs aresimultaneously connected to the summation point of the operationalamplifier 510.

In the context of the invention, the switching capacitor C530 always hasa capacitance of the same order of magnitude as the capacitance Csdefined between the measuring probe 100 and the reference probe 200,that is to say, preferably:

0.1Cs<C 530<10Cs

advantageously:

0.5Cs<C 530<5Cs

and highly advantageously:

Cs˜C 530

In an advantageous embodiment of the invention, the timebase 410generates a fixed frequency (f) at a duty factor of 50% delivering twoperiods (T1) and (T2) which are strictly identical (T1=T2) withf=1/(T1+T2).

Because the times T1 and T2 are strictly identical, the medium-termdrift in the frequency f generated by the oscillator 410 does not affectthe accuracy of the measurement.

The repetition frequency f of the measurement is typically of the orderof 5 to 50 kHz.

The signal available at the output of the integrator stage 500 can beused in various ways.

The structure of the output stage 600 shown in FIG. 1 will now bedescribed.

In the particular embodiment shown in FIG. 1, the output of theoperational amplifier 510 is connected via a resistor 602 to thenon-inverting input of an operational amplifier 604. This is configuredas a follower stage. To this end, the inverting input of the operationalamplifier 604 is connected to its output via a variable resistor 606.The inverting input of the operational amplifier 604 is also connectedto ground via a resistor 608.

The gain of the operational amplifier 604 can be tuned by the variableresistor 606, for example to adjust the output voltage of the circuit tothe required full-scale range.

The non-inverting input of the operational amplifier 604 also receives avariable voltage for the zero adjustment.

That voltage is formed from a voltage VREF which is preferably identicalto that delivered at the output of the power supply means 300. Thevoltage VREF is applied to the terminals of a potentiometer 610 whosevariable output drives the non-inverting input of an operationalamplifier 612. The latter has its inverting input connected to itsoutput which is connected to the non-inverting input of the operationalamplifier 604 via a resistor 614 of the same value as the aforementionedresistor 602. The very high impedance non-inverting input of theoperational amplifier 604 therefore receives a voltage of opposite signto the measuring slope, to enable zero adjustment by means of thepotentiometer 610.

Note that generating this voltage from the reference voltage VREFapplied to the. measuring probe 100 eliminates the effect of any driftin the reference voltage. This is because, if the reference voltage VREFshould drift, the voltage applied to the non-inverting input of theoperational amplifier 604 drifts in the same direction, so eliminatingany risk of the zero point drifting.

The output of the operational amplifier 604 can be connected to aprocessing stage 620, for example a 4-20 mA current loop or a signalprocessing stage for generating a spectrum characterizing the signal,for example to enable recognition of an analysed product.

One example of processing means for generating a spectrum of this typeis described hereinafter with reference to FIG. 4.

The embodiment shown in FIG. 2 will now be described.

The device shown in FIG. 2 includes a measuring probe 100, a referenceprobe 200, electrical power supply means 300, control means 400 and anintegrator stage 500 as described previously with reference to FIG. 1.

However, it is adapted to take account of any change in the permittivityof the medium between the measuring probe 100 and the reference probe200, for example for measurements in a tank that can contain differentproducts in succession. To this end, the device shown in FIG. 2 includesan auxiliary measuring probe 150, an auxiliary reference probe 250,auxiliary control means 450 and an auxiliary integrator stage 550.

The auxiliary measuring probe 150 and the auxiliary reference probe 250are designed to be placed in the same medium as the measuring probe 100and the reference probe 200. The auxiliary measuring probe 150 and theauxiliary reference probe 250 form compensator probes. By way ofnon-limiting example, and as shown in FIG. 8, the auxiliary measuringprobe 150 and the auxiliary reference probe 250 can be placed in thelower part of a tank containing a product whose level is to be measured.

The auxiliary reference probe 250 is at the same earth potential as thereference probe 200.

The auxiliary control means 450 include two two-way switches 452, 453controlled by the timebase 410 at the same timing rate as the switches420, 430, respectively.

Thus, if necessary, during the sequences T1 the switch 452 connects theoutput of the power supply means 300 to the auxiliary measuring probe150 via capacitive injectors 160 and connects the auxiliary measuringprobe 150 to the inverting input of an operational amplifier 560 whichis part of the integrator stage 550 during the times T2.

FIG. 2 shows that, if necessary, a resistive injector 162 can beinserted between the auxiliary measuring probe 150 and the auxiliaryreference probe 250, in a similar manner to the resistive injector 120.

The integrator stage 550 includes two capacitors: a reference capacitor562, analogous to the capacitor 520, connected between the invertinginput and the output of the operational amplifier 560, and a capacitor564 which has one terminal connected to ground and the other terminalconnected via the switch 453 to the output of the operational amplifier560 during the times T1 and the inverting input of the amplifier 560during the times T2.

The non-inverting input of the amplifier 560 is connected to the circuitground.

A calibration voltage VS2 proportional to the permittivity of theanalysed medium is obtained at the output of the operational amplifier560 at the output of the integrator stage 550.

This signal can be shaped in an output stage 650 receiving a zeroadjustment voltage from a stage 652 equivalent to the means 604, 610essentially described previously for the stage 600 with reference toFIG. 1.

Here the output of the stage 650 is used to monitor the gain of acontrol stage 660 also receiving the output of the shaping stage 600associated with the main probes 100 and 200. This to make themeasurement insensitive to variations in the permittivity of theanalysed medium.

The output of the stage 660 can be connected to any processing circuit620, for example, and as described with reference to FIG. 1, a 4-20 mAcurrent loop or a processing stage adapted to generate a spectrumcharacterizing the product analysed, for example.

The preferred embodiment of the present invention shown in FIG. 3 willnow be described.

FIG. 3 shows a measuring probe 100, a reference probe 200, power supplymeans 300, control means 400 and an integrator stage 500 as previouslydescribed with reference to FIG. 1.

However, the circuit shown in FIG. 3 neutralizes hysteresis of theaccumulated charge on the measuring probe 100 and discharges itcompletely in each cycle controlled by the control means 400.

To this end, as shown in FIG. 3, the non-inverting input of theoperational amplifier 510 is connected, not to the potential of thereference probe 200 (i.e. to ground), but to a potential of the oppositesign, relative to the potential of the reference probe 200, to thepotential applied by the means 300 to the measuring probe 100.

For example, for a duty factor of 1, i.e. for T1=T2, the electricalpower supply means 300 can apply a pulsed voltage of amplitude −E to themeasuring probe 100 and the non-inverting input of the operationalamplifier 510 can be connected to a voltage +E of the same amplitude asbut of the opposite sign to the aforementioned voltage −E.

To neutralize hysteresis, during the times T2 the measuring probetherefore receives a voltage which is the opposite of that appliedduring the times T1 via the switch 420 and the operational amplifier510.

More generally, for a duty factor p defined by the timebase 410, i.e.T1=p.T2, a potential +E2 is preferably applied to the non-invertinginput of the operational amplifier 510 and its absolute amplitude isequal to p times that of the supply voltage −E1 supplied by the circuit300 to the measuring probe 100. The average value of the voltage appliedto the measuring probe 100 is therefore a null voltage.

Note also that the resulting circuit naturally biases the inputoperational amplifier 510 by means of a continuous current flowingrelative to the virtual ground potential +E and enables the chargesampled by the switching capacitor 530 to be subtracted from thecontinuous current, which makes it possible to measure extremely smallcharges and to obtain an analog signal corresponding to a count of thecharges resulting from the integration of the signal, with no need forany output sample and hold.

In another embodiment of the present invention the non-inverting inputof the operational amplifier 510 can be connected to the circuit ground(i.e. to the potential of the reference probe 200) during the times Tland to the potential +E only during the times T2 and via a switchcontrolled by the timebase 410.

The structure of a spectrum recognition module. processing the signalfrom the integrator stage 500 to identify the analysed medium betweenthe measuring probe 100 and the reference probe 200 will now bedescribed with reference to FIG. 4.

FIG. 4 shows, in addition to the measuring probe 100 and the referenceprobe 200, electrical power supply means 300, control means 400 and anintegrator stage 500 as described previously with reference to FIG. 1 orFIG. 3.

The signal available at the output of the integrator stage 500 can beshaped in an output stage 600 receiving a reference voltage or zeroadjustment voltage from a stage 610.

The essential function of the additional processing means 700 shown inFIG. 4 is to amplify very strongly the signal from the upstream.stage600 in order to detect fluctuations of that signal and then to digitizeit and finally to calculate its spectrum. They include first of all astage 710 whose function is to transform the sampled signal from themeans 600 into squarewave signals whose amplitude is proportional to thepermittivity of the product analysed. The output of the stage 710 drivesthe input of a high-gain high-pass amplifier 712. The amplified signalis applied to a synchronous detector 714 adapted to restore theinformation relative to the reference potential of the module(electrical earth). The signal is then passed to an integrator 716 whosetime constant is very large compared to the sampling period. A feedbackbranch 718 connects the output of the integrator 716 and an input of theshaping stage 710.

A very strongly amplified and fluctuating signal is therefore obtainedat the output of the integrator 716.

The output of the integrator stage 716 is connected to a signalprocessing stage 720.

The processing stage 720 includes:

an analog/digital converter supplying digital values from the analogsignal from the integrator stage 716,

a control unit controlling storage means storing the digital values ofthe signal, and

calculating means, for example a DSP, a microcontroller or amicroprocessor.

The aforementioned calculating means:

perform recursive digital filtering,

calculate Fourier transforms and spectral densities in real time oroff-line with a number of points and a sufficient and fine resolution,using conventional windowing techniques (rectangular, Hamming, etcwindows) or overlapping techniques,

calculate crossed spectra in real time or off-line over sliding timeintervals,

calculate sliding averages in the time and frequency domain, and

calculate correlation and intercorrelation functions in the time orfrequency domain and if necessary search a digital database storingcharacterized signatures.

On the basis of Fourier transforms calculated in real time at timeintervals chosen according to the selected frequency resolution andresponse time (number of points, sampling frequency), the module 720searches the frequency band with the highest energy spectral density(ESD). Over a particular number No of consecutive points, the module 720calculates the sliding average of the ESD. In real time it compares theESD in a given frequency band to another one. Immediately a ratio K isreached, the ambiguity is resolved. The value No, the ratio K and thefrequency bands chosen depend on the substance analysed.

If necessary, to improve accuracy, the same calculation can be performedbut the crossed spectra calculated for two consecutive Fouriertransforms (Sc=(Sn−1)*(Sn*)) are processed rather than processing theFourier transforms directly.

When the spectrum has been obtained, the module 720 calculates and looksfor correlations with digital spectra stored in a database. The module720 can resolve the ambiguity on the basis of a correlation valuedefining an acceptable degree of confidence.

The measuring probe 100 and the reference probe 200 can be connected tothe processing means 500 by a two-wire cable, preferably a shieldedcable, or by a coaxial cable whose core is connected to the measuringprobe 100 and whose outer shield is connected to the reference probe200.

The present invention finds applications in many fields.

The following may be cited by way of non-limiting example:

measuring levels, for example the levels of:

electrically conductive or insulative liquids or powder materials, or

petroleum products such as liquefied petroleum gas (LPG), throughcontinuous level measurements or by sensing the level at particularmarkers, functioning for example as high-level and low-level detectors,regardless of the nature of the tanks concerned, for example whetherthey are made of metal or plastics material,

discrimination of products, for example:

determining the quality of oil in the automotive area,

in the area of petroleum products,

metering and monitoring product quality in the foodstuffs area, and

even in any other field, for example detecting a type of product in apipe, for instance discriminating between water and gas in a plasticsmaterial pipe, in particular a polyvinyl chloride (PVC) pipe,

person and intruder detection, and in particular:

intruder detection applied to the protection of objects of all kinds,

intruder detection applied in particular to protecting works of art,

intruder detection applied to protecting showcases in a shop,

sensing a person on an automobile seat to identify their position forintelligent control of various units of the automobile vehicle, such asairbags, the position of the steering column, the height of the steeringwheel or the orientation of the rear-view mirrors,

sensing persons or detecting intruders for alarm or countingapplications,

detecting objects for counting applications, and

sensing persons applied to a tactile keypad.

A series of applications of the present invention to level detectionwill now be described with reference to FIGS. 5 to 13.

FIG. 5 is a diagrammatic representation of an embodiment in which thereference probe 200 is an electrically conductive, for example metal,tank 10 containing an electrically conductive or insulative liquid orpowder whose level is to be measured. In this case the measuring probe100 is a conductive body inside the tank 200, for example a circularsection rod, and is preferably positioned vertically so that it dipsinto the medium to be analysed. The measuring probe 100 preferably has aconstant cross section throughout its length. The measuring probe 100must of course be separated from and spaced from the reference probe 200formed by the tank. The measuring probe 100 and/or the reference probe200 must be coated with an electrically insulative coating if theproduct to be analysed contained in the tank 200 is electricallyconductive.

The measuring probe 100 and the reference probe 200 are connected to theprocessing and analysis means 500 previously described by connectingmeans 20, 22.

By way of non-limiting example, for a cylindrical measuring probe 100,the voltage obtained at the output of the integrator stage 500 is of theform:${VS} = {\left( {{E \cdot \eta \cdot 2 \cdot \pi \cdot ɛ}\quad {c \cdot G \cdot H}} \right)/\left( {\left\lbrack {{LOG}\quad \frac{\left( {R + e} \right)}{R}} \right\rbrack + \frac{ɛ\quad {m \cdot e}}{ɛ\quad {c \cdot R}}} \right)}$

in which:

η=0.5=T2/(T1+T2) is the duty factor neutralizing the residual hysteresisof charges,

εm is the permittivity of the liquid analysed,

εc is the permittivity of the coating of the probe,

G is the gain of the system,

R is the radius of the measuring probe 100,

e is the thickness of the insulative coating on the measuring probe 100,

H is the depth of the conductive liquid or the powder, and

E is the value of the applied field.

FIG. 6 shows an embodiment in which the reference probe 200 is not thetank 10 containing the fluid to be analysed but an electricallyconductive material cylinder surrounding the measuring probe 100 andincorporating apertures to enable direct fluid communication between theinterior volume of the tank 10 and the interior volume of the referenceprobe 200 accommodating the measuring probe 100.

Of course, the measuring probe 100 and/or the reference probe 200 mustbe coated with an electrically insulative material which is not porousto the fluid to be analysed if the fluid is electrically conductive.

FIG. 7 shows an embodiment in which the reference probe 200 is anelectrically conductive body disposed in the tank 10 and at leastsubstantially parallel to the measuring probe 100.

FIG. 8 shows an embodiment compatible with the processing circuit shownin FIG. 2.

FIG. 8 shows a measuring probe 100 and a reference probe 200 (here as inthe FIG. 7 embodiment, although it could conform to any other embodimentof the invention), associated with two electrodes 150, 250 which arelocated near the bottom of a tank containing a fluid to be analysed sothat they are always immersed in it. They are connected to theintegrator stage 550 by electrically insulated connections.

FIG. 9 shows an embodiment designed to provide binary detection of highand low levels.

FIG. 9 shows a reference probe 200 connected to the electricallyconductive material tank 10 (but which could instead be a body separatefrom the tank and placed therein), and two measuring probes 100, 100′ atthe high and low levels to be detected, respectively. The two measuringprobes 100, 100′ are successively connected to power supply means and torespective integrator stages 500 by respective two-way switches 420 asdescribed previously with reference to FIG. 1.

The output voltage of the associated integrator stage 500 varies if thefluid analysed is above or below the level of the measuring probe 100 or100′ concerned. Consequently, it is sufficient to compare the outputsignal of the integrator stages 500 and a reference signal to determinewhether the high or low level in the tank has been reached.

Of course, there could instead be a single measuring probe detectingonly the high level or only the low level, for example, or more than twomeasuring probes staggered along the height of the tank to detectrespective levels.

FIG. 10 shows an embodiment including two measurement probes 100 in atank 10 and respectively at a high level and a low level to be detectedand a reference probe 200 in the tank and near its bottom.

FIG. 11 shows an embodiment which is preferably used to identify theproduct analysed, for example, to measure the quality of the oil in anautomobile vehicle or to discriminate the quality of food products orpetroleum products. FIG. 11 shows a measuring probe 100 comprising avertical electrically conductive material rod 102 which terminates atthe bottom in a circular ring 103. The vertical rod 102 of the measuringprobe 100 is covered with an electrically insulative material 104 whichis in turn covered by a reference probe 200. Likewise the ring 103.However, at the level of the bottom ring 103 the reference probe 200 isspaced from the insulation 104 (or the measuring probe 100 is spacedfrom the insulation 104) by a distance d to allow the fluid to beanalysed to enter the space defined in this way between the insulation104 and the reference probe 200 or the measuring probe 100.

Thus only the lower parts of the measuring probe 103 and the referenceprobe 200 are actively involved in discriminating the product to beanalysed.

Of course, a simple detector device like that shown in FIG. 5 can beused to discriminate or identify an analysed product through directprocessing of the signal obtained representative of the permittivity ofthe medium between the two probes 100 and 200, instead of using it todetect a level.

FIG. 12 shows an embodiment for detecting levels, similar to that shownin FIGS. 9 and 10, in which the measuring probes 100 are outside thetank 10. This embodiment typically applies to a tank formed of anelectrically insulative material.

FIG. 13 shows another embodiment including a temperature sensor 480 forapplying compensation to the measurement.

FIG. 13 shows a measuring probe 100 and a reference probe 200 of thetype shown in FIG. 6. The invention is not limited to this embodiment,however. It can be applied to any other geometry and arrangement of theprobes according to the invention.

Moreover, the temperature probe shown in FIG. 13 is placed over themeasuring probe 100. The temperature probe 480 can instead be placedanywhere else, however.

The output signal of the temperature probe 480 shown in FIG. 13 is usedto correct the value of the voltage −E applied to the measuring probe100 during the times T1 and to define a correction voltage applied tothe input of the integrator stage 500 during the same times T2.

In this case, the voltage −E is obtained at the output of an operationalamplifier 481 whose non-inverting input is connected to the output ofthe temperature sensor 480 by a resistor 482 and to a fixed referencevoltage via a resistor 483.

The inverting input of the operational amplifier 481 is connected toground by a resistor 484 and to its output by a resistor 485.

There is therefore obtained at the output of the amplifier 481 a voltage−E which varies in the opposite direction to the temperature, socompensating changes with temperature of the permittivity of someliquids.

The output of the amplifier 481 is connected to the measuring probe 100via the aforementioned two-way switch 420.

The correction voltage is sampled by an inverting amplifier 486 at thecursor of a potentiometer 487 between the output of the amplifier 481and ground.

The output of the inverting amplifier 486 is connected to the input ofthe integrator stage 500 during the time T2 via a switch 488 controlledby the timebase 410.

A circuit applying a temperature correction by means of components 480to 488, as shown in FIG. 13, can be used in a simple application to thediscrimination of a product without measuring the level.

In a further embodiment of the invention, to apply a correction as afunction of the measured permittivity of the medium, the feedbackapplied to the operational amplifier 481 via the resistor 482 can comefrom a processing stage receiving a signal from a measuring probesimilar to the probe 150 shown in FIG. 8, instead of from a temperaturesensor.

As a general rule, in the context of the present invention, the initialoffset voltage can be compensated in two ways: a) by neutralizing theinitial offset by means of an amplifier following on from the capacitorswitching system 600 and by applying a voltage of the opposite sign tothe field applied to the summation point of the operational amplifier510, or b) by applying synchronously to the summation point of thecapacitor switching amplifier 600, during the counting of the charges, avoltage of opposite sign to the applied field.

FIG. 14 shows an embodiment for detecting or discriminating products,for example for discriminating between solid, liquid or gas products,such as water or gas, in a non-conductive duct 30. To this end, FIG. 14shows a measuring probe 100 and a reference probe 200 placed on the wallof the duct 30, for example in a diametrically opposed arrangement,although this is not limiting on the invention. The measuring probe 100and the reference probe 200 can be shifted axially or at angularpositions that are not diametrically opposed. If necessary the probes100 and 200 are protected from the media flowing in the duct 30 by acoating which is sealed against those media. A device of this kind canin particular detect the presence of water in a gas.

The integrator stage 500 forming a charge counting system delivers atits output a voltage proportional to the permittivity of the productbetween the electrodes 100 and 200. The ratio between the permittivityof water and the permittivity of most gases being greater than 15, suchmeans can easily discriminate the presence of water or gas in the duct.

FIG. 15 shows an embodiment applied to intruder detection. In this case,the measuring probe and the reference probe are conductive bodies, forexample electric wires, which are routed along an area undersurveillance. By way of non-limiting example, the distance between thetwo probes 100 and 200 can be of the order of 5 cm.

More generally, in the context of the present invention, the distancebetween the two probes 100 and 200 is typically from 1 to 10 cm andpreferably of the order of 5 cm.

Non-significant areas of the bodies constituting the probes 100, 200 canbe partly electrically insulated or shielded over their length. Suchlocal insulation or shielding can be achieved by surrounding themeasuring probe 100 locally with a conductive sheath 40 referred to thepotential of the reference probe 200. Any movement of a person or objectin the environment of the probes 100, 200 modifies the permittivity ofthe medium and therefore varies the output voltage of the integratorstage 500 and so the intrusion is detected. For example, the circuitaccording to the invention can detect movement at up to 40 cm from theprobes with a voltage of the order of 4 volts between the measuringprobe 100 and the reference probe 200.

It has been found that the environment can eventually generate a driftor offset voltage as a function of ambient temperature and relativehumidity. As shown in FIG. 15, this can be corrected with the aid of amodule 730 which calculates the average value of the drift and correctsthe signal accordingly. In this context, a distinction can be drawnbetween intrusion into, addition of an object to and removal of anobject from the environment of the probes 100 and 200, according to thepositive or negative sense in which the detected voltage at the outputof the stage 500 changes.

Correction based on the change of the ambient permittivity as a functionof relative humidity and temperature in particular can instead beobtained with the aid of a reference signal generated by an additionalintegrator stage controlled by additional measuring and reference probesplaced in the same environment as the detector measuring and referenceprobes, but at a location that is not accessible to an intruder andtherefore not responsive to such influence.

A variant of the device shown in FIG. 15 can be used to protect apainting or work of art or the like.

For this it is sufficient to place the two probes 100 and 200 near thework to be protected so that any movement of the work or entry into theenvironment of the probes generates a variation of the output signal ofthe associated integrator stage 500.

Of course a respective pair of probes 100, 200 dedicated to each objectunder surveillance can be provided, or one pair of probes 100, 200 canbe associated with a set of objects under surveillance, for example in ashowcase or at a demonstration or exhibition. In the latter case it issufficient for the two probes 100, 200 to be long enough to encompassall of the environment of the objects in question. The probes 100, 200can be under or behind the support for the objects under surveillance.Once again, as for the other embodiments of the present invention, theprobe connecting wires, which have no detection function, must beneutralized to prevent false alarms. Moreover, the location of theobjects, for example a table, display unit or showcase, must not bemetallic or include electrically grounded conductive structures.

FIG. 16 is a diagrammatic representation of an embodiment of the systemfor detecting the presence and/or the position of a user on a seat 50,for example a vehicle or aircraft seat. A system of this kind can beused to detect the position, direction or presence of a user, forexample, in order to provide intelligent control of an airbag in theevent of a collision.

In this case, the seat is preferably equipped with several pairs ofmeasuring probes 100 and reference probes 200 disposed under theupholstery of the seat, for example, facing characteristic areas, forexample the legs, back, shoulders and head. The outputs from the probescan be directed to respective processing stages or to a commonprocessing stage via a multiplexer.

The signals delivered by the probes can be processed in many differentways. By way of non-limiting example, a computer can sum the signalsfrom each of the probes, applying a particular weighting.

A device of this kind can distinguish a child from an adult, forexample, and control an airbag accordingly, to avoid injuring the user.

FIG. 17 shows an embodiment for detecting the passage of persons orobjects through a doorway 60, for example for counting or alarmpurposes.

In this case, the measuring probes 100 and the reference probe 200 areparallel to a wall of the doorway 60, for example a vertical lateralwall. By way of non-limiting example, the two probes 100, 200 can takethe form of electrical wires extending the full height of the doorway 60and separated from each other by a distance of the order of 5 cm. Thedoorway 60 preferably includes no other earthed electrically conductive,in particular metal, structures.

The same type of device can be used to detect the passage or movement ofobjects or to count them. By way of non-limiting example, a measuringprobe 100 and a reference probe 200 can be used for this, placed onrespective opposite sides of a conveyor 70 along which the objects move,as shown in FIG. 18, or juxtaposed along the conveyor.

FIG. 19 is a diagrammatic representation of an embodiment of theinvention which takes the form of a keypad. Each of the keys 80 isformed by two areas of an electrically conductive material, respectivelyforming a measuring probe 100 and a reference probe 200, disposed on asupport 82, preferably under a screenprinted electrically insulativescreen, formed of a sheet of plastics material, for example. The twoprobes 100 and 200 can be a few millimetres apart, for example. Thepermittivity of the medium surrounding the probes 100 and 200 varieswhen the user's fingers approach the corresponding detection areasmaterialized by the probes, which varies the output level of anassociated charge integrator stage. The probes 100 and 200 are connectedto the processing means 500 by any connecting means 20, 22. Theconnecting means formed in this way must be neutralized outside therequired detection areas by shielding them to prevent spurious detectionif the user's fingers approach these connecting areas. The referenceprobes 200 are preferably all connected together. Each key formed by apair of probes 100 and 200 can be connected to a respective integratorstage 500. However, the various keys are preferably connected to acommon integrator stage via a multiplexer. In this case, each key isscanned successively by the aforementioned sequences T1, T2 and a changein the output signal of the integrator stage 500 is attributed to thesynchronously scanned key.

FIG. 20 shows another embodiment of the present invention, forming apressure sensor. In this case, the two probes 100 and 200 are placed onrespective parts of a sensor capable of relative movement due to theeffect of applied pressure. As shown in the figure, the sensor 84 has ahousing 85 divided into two chambers 86, 87 by a flexible membrane 88which is deformable due to the pressure fed into a first chamber 86 viaan inlet 89. One of the two probes, here the measurement probe 100, isplaced on the deformable membrane 88 and the other, here the referenceprobe 200, is placed on a fixed wall of the sensor housing, for examplethe bottom wall of the second chamber 87, or vice versa. The secondchamber 87 can be closed or vented to the atmosphere or communicate witha reference pressure. The deformable membrane 88 can be associated witha calibrated load spring or not. The probes 100 and 200 are connected toan integrator stage 500 by connecting means, for example a flexibleconductor in the case of the probe on the deformable membrane. Theoutput voltage of the integrator stage 500 associated with the sensorshown in the figure varies in inverse proportion to the distance betweenthe probes 100 and 200 reflecting variations in pressure.

FIG. 21 shows another embodiment of the present invention, forming asensor responsive to the inflation of a tyre 90. In this case ameasuring probe 100 and a reference probe 200 are placed in respectiveareas of the tyre or the associated structure so that they move relativeto each other depending on the inflation of the tyre. By way ofnon-limiting example, the measuring probe 100 is the radial metal casingof the tyre 90 and the reference probe 200 is the rim or a metal stripapplied to the rim 92 and insulated from it by an electricallyinsulative material, such as an elastomer, or vice versa. The probes 100and 200 are connected to an integrator stage 500 on the rim byconnecting means, for example a shielded connector 20/22. The distancebetween the two probes 100 and 200 varies according to the inflation ofthe tyre. This therefore applies also to the output signal of theintegrator stage 500. The corresponding information is transmittedbetween the rim and the hub of the vehicle, or more generally itsbodywork, by connecting means such as an electromagnetic transponder oran optical connection.

Of course, the present invention is not limited to the embodimentspreviously described, but encompasses all variants conforming to thespirit of the invention.

The present invention has many advantages over prior art measuringdevices.

In particular it provides a very sensitive circuit that can measurecapacitance values as low as a few hundred femtocoulombs, for example.

The present invention can also be used to implement detector means withperfect galvanic isolation, which are therefore totally safe for theuser. All that is required is to protect the probes 100 and 200 with atotally electrically insulative screen, for example a tactile screen. Adevice of this kind finds an application in the management of utilitiesin a sensitive environment such as a bathroom, for example. In this casethe invention takes the form of a panel made up of various areas or keyseach of which is associated with a respective measuring sensor 100 tocontrol a specific function, for example valve open/shut, flowrateadjustment, hot water, cold water, or any equivalent function, forexample by means of an electromechanical device.

What is claimed is:
 1. Measuring device employing indirect measurementof permittivity and including two electrically conductive bodiesrespectively constituting a measuring probe (100) and a reference probe(200), electrical power supply means (300) adapted to deliver a DCelectrical voltage of controlled amplitude, an integrator stage (500)including a capacitor switching system (530) and control means (400)adapted to define a cyclic series of two sequences at a controlledfrequency, namely a first sequence (T1) during which the electricalpower supply means (300) are connected to the measuring probe (100) toapply an electric field between the measuring probe (100) and thereference probe (200) and accumulate electrical charge on the measuringprobe (100) and a second sequence (T2) during which the electrical powersupply means (300) are disconnected from the measuring probe (100) whichis connected to a summation point of the integrator stage (500) totransfer its charge into the integrator stage (500) and obtain at theoutput thereof a signal representative of the permittivity between themeasuring probe (100) and the reference probe (200), characterized inthat the integrator stage (500) includes an operational amplifier (510),a first integrator capacitor (520) constituting a feedback capacitor ofsaid amplifier (510) and a second capacitor (530) switched between theoutput and the input of the operational amplifier (510) at the timingrate of the sequences (T1, T2) controlled by the control means (400), sothat under steady state conditions there is obtained at the output ofthe operational amplifier (510) an “equilibrium” voltage V_(S) equal to−E·Cs/C530 where −E is the amplitude of the voltage at the terminals ofthe electrical power supply means (300), Cs is value of the capacitordefined between the measuring probe (100) and the reference probe andC530 is the value of the second switched capacitor (530).
 2. Deviceaccording to claim 1 characterized in that the switched capacitor (530)has a capacitance of the same order of magnitude as the capacitancebetween the measuring probe (100) and the reference probe (200). 3.Device according to claim 1 characterized in that the integratorcapacitor (520) is connected between the inverting input and the outputof the operational amplifier (510).
 4. Device according to claim 1characterized in that the switched capacitor (530) is connected to theoutput of the operational amplifier (510) during the sequences (T1) ofsupply of power to the measuring probe (100) and to the input of theoperational amplifier (510) during the sequences (T2) of connection ofthe measuring probe (100) to the input of that amplifier (510). 5.Device according to claim 1 characterized in that the switched capacitor(530) is switched simultaneously with the measuring probe (100). 6.Device according to claim 1 characterized in that it includes means forapplying a null average voltage to the measuring probe (100).
 7. Deviceaccording to claim 6 characterized in that the operational amplifier(510) receives on a second input opposite that adapted to be connectedsequentially to the measuring probe (100) a voltage (+E) of oppositesign to the voltage (−E) applied by the power supply means to themeasuring probe.
 8. Device according to claim 7 characterized in thatthe absolute value of voltage (+E2) applied to the second input of theoperational amplifier (510) is equal to p times the amplitude of thesupply voltage (−E1) delivered by the power supply means (300) to themeasuring probe (100).
 9. Device according to claim 1 characterized inthat the control means (400) are adapted to define a duty factor of 50%,i.e., two successive sequences (T1, T2) of the same duration.
 10. Deviceaccording to claim 1 characterized in that the control means (400) areadapted to apply a stepped voltage to the measuring probe (100). 11.Device according to claim 1 characterized in that the control meansdefine sequence cycles (T1 and T2) at a frequency from 5 to 50 kHz. 12.Device according to claim 1 characterized in that the capacitor (520)constituting a feedback capacitor of the operational amplifier (510) hasa capacitance at least 1000 times that of the switching capacitor (530).13. Device according to claim 1 characterized in that the integrator stage is connected to the input of a processing stage (600) including zeroadjustment means (610, 612, 614).
 14. Device according to claim 1characterized in that the integrator stage (500) is connected to aprocessing stage (600) including full scale deflection adjustment means(606).
 15. Device according to claim 1 characterized in that it furtherincludes at least one auxiliary measuring probe (150) used to determinethe permittivity of the environment of the measuring and referenceprobes (100, 200) and to apply a correction to the processing of thesignal.
 16. Device according to claim 15 characterized in that theauxiliary measuring probe (150) is near the bottom of a tank.
 17. Deviceaccording to claim 15 characterized in that the correction means (150,480) are adapted to modify the gain of a control stage (660).
 18. Deviceaccording to claim 1 characterized in that it further include atemperature probe adapted to measure the temperature of the environmentof the measuring probe (100) and the reference probe (200).
 19. Deviceaccording to claim 1 characterized in that the correction means (150,480) are adapted to apply compensation to the voltage applied by thepower supply means (300) to the measuring probe (100).
 20. Deviceaccording to claim 1 characterized in that the correction means (150,480) are adapted to generate a correction voltage applied to the inputof the integrator stage (500).
 21. Device according to claim 1characterized in that it includes means for compensating an initialoffset voltage of the integrator stage (500).
 22. Device according toclaim 21 characterized in that the offset voltage compensating meansinclude an amplifier downstream of the capacitor switching system (600)adapted to apply a voltage of opposite sign to the field applied to thesummation point of the integrator stage (500).
 23. Device according toclaim 21 characterized in that the offset compensating means includemeans adapted to apply a voltage of opposite sign to the applied fieldsynchronously to the summation point of the capacitor switchingamplifier (600) during counting of charges.
 24. Device according toclaim 1 characterized in that it further includes means for generating asignal representative of the spectrum of the medium surrounding themeasuring probe (100).
 25. Device according to claim 24 characterized inthat the spectrum analyser means include Fourier transform calculatingmeans and means for comparing the spectra obtained with digital spectrastored in a database.
 26. Device according to claim 24 characterized inthat the spectrum analyser means include a shaping stage (710), ahigh-gain high-pass stage (712), a synchronous detector (714), anintegrator (716) and a calculation stage (720).
 27. Device according toclaim 1 characterized in that it includes two electrically conductivebodies (100, 200) respectively forming a measuring probe and a referenceprobe at distances from 1 to 10 cm, preferably of the order of 5 cm. 28.Device according to claim 1 characterized in that the measuring probe(100) and/or the reference probe (200) has an electrically insulativecoating sealed against the medium surrounding the probe.
 29. Deviceaccording to clam 1 characterized in that at least some sections of theconnecting means (20, 22) connecting the measuring probe (100) areneutralized by an electrically conductive materials sheath at the samepotential as the reference probe (200).
 30. Device according to claim 1characterized in that it is adapted to implement a function chosen fromthe group comprising measuring a level, discriminating products ordetecting presence or intrusion.
 31. Device according to claim 1characterized in that it includes a plurality of measuring probes (100)connected to respective integrator stages (500).
 32. Device according toclaim 1 characterized in that it includes a plurality of measuringprobes (100) connected to a common integrator .stage (500) via amultiplexer.
 33. Device according to claim 1 characterized in that inconstitutes a level measuring device.
 34. Device according to claim 33characterized in that the measuring probe (100) is generally vertical ina tank (10) and the integrator stage (500) is connected to processingmeans (600) adapted to generate a signal representative of the level offluid in the tank (10).
 35. Device according to claim 33 characterizedin that the measuring probe (100) is formed by an electricallyconductive area in a tank (10) at a height corresponding to a scanninglevel.
 36. Device according to claim 33 characterized in that itincludes a plurality of measuring probes (100) at respectivediscrimination levels in a tank (10).
 37. Device according to claim 33characterized in that the fluid to be detected is electricallyconductive and the measuring probe (100) and/or the reference probe(200) has an electrically insulative coating.
 38. Device according toclaim 1 characterized in that the reference probe (200) is forced by thetank.
 39. Device according to claim 1 characterized in that thereference probe (200) is an apertured member around the measuring probe(100).
 40. Device according to claim 1 characterized in that itconstitutes a device for discriminating a product flowing in a pipe (30)equipped with a measuring probe (100) and a reference probe (200). 41.Device according to claim 1 characterized in that it includes ameasuring probe (100) and a reference probe (200) which run along anarea under surveillance to form an intruder detector device.
 42. Deviceaccording to claim 1 characterized in that it includes a plurality ofpairs of measuring electrodes (100) and reference electrodes (200)arranged on a seat to detect the presence and/or the position of a user.43. Device according to claim 1 characterized in that it includes ameasuring sensor (100) and a reference sensor (200) running along a wallof a doorway (60) to form a passage detector.
 44. Device according toclaims 1 characterized in that it includes a measuring probe (100) and areference probe (200) placed at the side of a conveyor belt to detectthe passage of objects.
 45. Device according to claim 1 characterized inthat it includes a plurality of pairs of measuring probes (100) andreference probes (200) forming a tactile keypad.
 46. Device according toclaim 1 characterized in that the measuring probe (100) or the referenceprobe (200) is placed on a deformable member (88) to form a pressuresensor.
 47. Device according to claim 1 characterized in that themeasuring probe (100) and the reference probe (200) are placed on twoareas of a tyre which can move relative to each other as a function ofthe inflation of said tyre to form a tyre inflation sensor.